Extraction of prior grain boundaries from interfaces of martensite based on particular statistics for inter-variant disorientations

The paper considers grounds and limitations of various ways to reveal prior austenite grain boundaries (PAGB) in martensite in terms of EBSD data and a predetermined orientation relationship (OR). In order to improve this reconstruction, a novel approach is proposed that extracts PAGB from a network of interfacial disorientations rather than orientations of transformed crystals. Thus, errors due to non-uniformity of deformed austenite grains are excluded while essentially diminishing computational expenses. Special attention is paid to special (=3) twin PAGB because a notable part of then would be lost when conventionally discarding all combinations (V1/Vi, i=2,3,…,24) of OR variants. Instead a limited set (i=2-6 and 8) of variant couples most frequent in martensite is considered. Although such a procedure occasionally admits wrong segments of PAGB and still partly discards their twin parts, a simple correction of such errors is possible owing to very low probability to meet them at successive segments of prior boundaries. The algorithm is verified on low carbon martensitic steel where actual PAGB have been previously imaged by the thermal etching in vacuum. In order to quantify their correspondence to PAGB reconstructed by means of EBSD, related length distributions in random intersection of prior grains by straight lines are analyzed.